Plasma arc torches are commonly used when fabricating metal structures. They are frequently employed in operations such as cutting, welding, treating surfaces, and annealing. These torches include an electrode which supports an arc which is struck between the electrode and the workpiece. The arc is sustained therebetween when the torch is in the arc transfer mode of operation. The electrodes currently used in many plasma arc torches have an electrode tip body fabricated from a high conductivity material such as copper or a copper alloy. An emissive insert is placed in the tip of the electrode. These emissive inserts are fabricated from materials with a low work function, and are frequently made from hafnium, zirconium, or tungsten. Further discussion of these electrodes is found in U.S. Pat. No. 5,023,425, which also notes that such electrodes have short lives, since the arc may become supported from the tip body of the electrode, rather than from the emissive insert, causing deterioration of the tip body. To extend the life of such electrodes, the '425 patent teaches providing a sleeve having a radial thickness of at least about 0.01 inches positioned between the emissive insert and the copper or copper alloy tip body of the electrode. This sleeve is fabricated from a metallic material having a work function which is greater than that of the material of the emissive insert, and maintains the arc being supported by the emissive insert. The '425 patent suggests the use of a variety of noble metals for the sleeve, and provides examples of electrodes employing silver sleeves.
The '425 patent teaches a method for fabricating such an electrode which includes the following steps. A first blank of copper or copper alloy is provided, having a front face. A cavity is bored in the front face. A second blank of silver is formed and is metallurgically bonded into the cavity of the first blank by use of a brazing material. An opening is then drilled into the second blank, and an emissive insert is force fitted into the drilled opening. The assembly is then machined to provide a smooth front face for the assembly.
U.S. Pat. No. 5,200,594 teaches an alternative solution to the problem of short electrode life by employing a plated emissive insert. The emissive insert is first plated with nickel and thereafter plated with a noble metal.
While the solutions of both of the above mentioned patents provide an electrode with a longer life, they complicate the process of fabricating the electrode. Furthermore, the electrodes of the '425 patent require substantial quantities of silver to fabricate, much of which is machined away during the manufacturing of the electrode. The method of the '594 patent reduces the quantity of the high work function metal which is required; however, it requires a double plating process where an intermediate layer of nickel is deposited onto the insert before plating with a noble metal. Furthermore, the '594 patent teaches that the use of a nickel plate is required to assure adhesion of plated layers during the subsequent processing. The adhesion is reported to be important to maintain a good thermal path for dissipating the heat generated in use, thereby extending the life of the electrode.
Thus, there is a need for an electrode which can be readily fabricated with little waste and which will provide a long service life.